Centrelink is deliberately ripping-off thousands of Australians caught up in its data matching "robo-debt" program, with managers telling public servants at the agency to enforce debts they know are bogus, according to explosive new claims.

The serious allegations have emerged as a second Liberal backbench senator breaks ranks and criticises the data matching effort which the government hopes will add $4.5 billion to the budget bottom line.

Another whistleblower has come forward with an insider's account of how the "debts" are being pursued, alleging that glaring errors are deliberately ignored by Centrelink to allow it to extract money from its clients while others are being told they must repay "fictitious payments".